For metering and protection purposes it is necessary to measure currents in high voltage transmission lines. The current measurement is usually performed at the high voltage potential of the line while the gauging instrument comprises parts that are at or near earth potential, which require complicated insulation. Typically the equipment must work both for a single conductor and for a plurality of conductors carrying currents of different magnitudes and phases. Current shunts, for example, can be used to measure such a current. They may comprise an impedance series-connected with the conductor and the current can be estimated from the voltage across the impedance. Again, the measurement requires insulation between parts of the gauging instrument at or near earth potential and the impedance at high voltage potential.
Alternatively the current may be determined indirectly from the magnetic field generated by the conductor. Electrical contact with the conductor is not necessary which has the advantage that the measurement can be performed at or near earth potential. This method involves typically the use of a plurality of separate transformers and the measurement of the induced currents. The transformers may comprise iron or air cored toroidal coils surrounding the conductor carrying the current. These transformers can be manufactured with sufficient accuracy and linearity over a limited range, are relatively insensitive to magnetic fields generated by other nearby currents and can also provide energy for the operation of the equipment connected to them. Since toroidal coils surround the conductors, however, they require insulation between the conductor potential and earth potential. Because of the cost associated with this insulation such current transformers are relatively expensive.
Hall effect devices and coils mounted at or near earth potential are also used to measure the magnetic field generated by currents in a group of conductors. These techniques have the significant disadvantage that they detect the total magnetic field at their location. This may be influenced by a number of nearby conductors and it may be impossible to measure the magnetic field associated with only one particular conductor within the group. In three phase power systems, when a sensor is separated from a conductor by a safe working distance, which will prevent flashover under any circumstances, the other conductors are nearby and their contribution to the total magnetic field is significant. Hall effect devices and coils mounted at or near earth potential also have to cope with very low magnetic fields and therefore low signal strengths. The Hall effect and iron-cored coils also have linearity problems over large dynamic ranges.
An alternative method was proposed by Blatt (WO 89/09411). This documents discloses the measurement of the magnetic fields near every conductor within a group of conductors using a plurality of coils. A computer routine may then be employed to calculate the contribution of the magnetic fields of each of the neighbour conductors to that of a particular conductor. This method, however, is quite involved, as it requires the measurement of the fields near all conductors in the vicinity and also calibration.
It is therefore desirable to be able to provide an improved method and apparatus for determining, at earth potential, the current in a conductor.